Using back-to-back connected SCRs as a static switch in between an AC power supply and a load is quite common nowadays. For example, please refer to FIG. 1, it shows a schematic circuit diagram of a conventional tap changing transformer including six pairs of back-to-back SCRs (SCR1 to SCR6) for line voltage regulation (with input supply: 240-520V and output supply: 220+/−10% V). In which, each pair of the back-to-back SCRs has a first terminal (k1(1) to k1(6)) and a second terminal (k2(1) to k2(6), and each of the SCRs has a gate (G1(1) to G1(6) and G2(1) to G2(6)). In such an application of SCRs, a commutation failure detection circuit is required to avoid a dramatic damage to the transformer caused by a malfunction of the SCRs.
Please refer to FIG. 2, which shows a block diagram of a conventional commutation failure detection circuit using bridge and opto-coupler for a back-to-back SCR circuit. In which, it shows a back-to-back SCR circuit and the failure detection circuit including a resistor bank coupled to the back-to-back SCR circuit, a full-bridge circuit coupled to the resistor bank and the back-to-back SCR circuit, and an opto isolator having an opto-coupler (not shown) coupled to the full-bridge circuit, receiving a DC supply and outputting an output.
Assume that the first SCR of a SCR pair of the back-to-back SCR circuit (not shown) is triggered at some firing angle in a positive half cycle of an AC input voltage (not shown) of the back-to-back SCR circuit. A forward voltage is present across the SCR pair until any one of the SCR starts conducting. This forward voltage is applied to the full-bridge rectifier through the resistor bank. The resistor bank serves the purpose of limiting the current flowing into the full-bridge circuit. The rectified voltage turns on a light emitting diode (not shown) in the opto-coupler and this signal is transmitted to the output.
When any one of the SCR pair starts conducting, the voltage across the opto-diode is zero. Therefore, there is no signal on the opto-coupler output. A similar operation follows in the negative half cycle of the AC input voltage.
The advantages of the aforementioned conventional failure detection circuit using bridge and opto-coupler are that the components count of which is relatively low. And the disadvantages of the aforementioned conventional failure detection circuit using bridge and opto-coupler are that the total applied voltage is dropped across the series-connected resistor bank around the zero crossing of the AC input voltage. Therefore, a forward current available to turn on the full-bridge including four diodes builds up slowly. Thus, there is always some delay present in commutation failure detection of the SCR.
Please refer to FIG. 3, which shows a block diagram of a conventional commutation failure detection circuit using opto-couplers for a back-to-back SCR circuit. In which, it shows a back-to-back SCR circuit and the failure detection circuit including a resistor bank coupled to the back-to-back SCR circuit and two opto isolators each having an opto-coupler (not shown) and outputting an output, one opto isolator is coupled to the resistor bank, and the other is coupled to the back-to-back SCR circuit and receives a DC supply voltage.
Assume that the first SCR of a SCR pair of the back-to-back SCR circuit (not shown) is triggered at some firing angle in the positive half cycle of the AC input voltage (not shown) of the back-to-back SCR circuit. The forward voltage is present across the SCR pair until any one of the SCR starts conducting. A maximum voltage drop appears across the series resistor bank, which serves to limit the current flowing into the opto-couplers. One opto-coupler is connected in reverse direction to allow conduction during a negative half-cycle of the AC input voltage. An input voltage of each of the opto-couplers turns on an opto-diode (not shown) presents in each of the opto-couplers and transmits a signal to the opto-coupler output.
When any one of the SCR pair starts conducting, the voltage across the opto-diode is zero. Therefore, there is no signal at the opto-coupler output. A similar operation follows in the negative half cycle of the AC input voltage.
The advantages of the above-mentioned conventional failure detection circuit using opto-couplers are that the components count of which is also relatively low. But the disadvantages of the above-mentioned conventional failure detection circuit using opto-couplers are that the total applied voltage is dropped across the series-connected resistor bank around the zero crossing of the AC input voltage. Therefore, the forward current available to turn on the opto-diodes builds up slowly. Thus, there is always some delay present in commutation failure detection of the SCR.
Keeping the drawbacks of the prior arts in mind, the applicant proposes a commutation failure detection circuit for a back-to-back SCR circuit and the controlling methods thereof having relatively better efficiency.